1. Field of Invention
The present invention relates to a plating method and apparatus for conductive metal base materials, and a method for manufacturing three-dimensional metal objects.
2. Brief Description of the Related Art
Various types of products using plated metal base materials such as iron and aluminum exist. For example, bumpers for automobiles, rearview mirrors, reflectors, electric and electronic parts, precision instrument parts, aircraft components, engine pistons, bus bars, and electrical wires are included in such products.
Generally, plating a metal base material such as aluminum includes a pretreating stage and a plating stage. In the pretreating stage, oxide film and dirt are removed from the surface of the base material to ensure adhesion between the base material and plated layer. A zincate process is employed for the pretreating stage. The zincate process includes a degreasing step, an etching step, an acid wash step, and a zinc displacement step, all of which are performed on the surface of the base material.
In the degreasing step, the surface of the base material is degreased. In the etching step, the surface of the aluminum base material is eroded by an etching solution. In the acid wash step, the surface of the aluminum is eroded by an acid such as nitric acid, hydrofluoric acid, or sulfuric acid. In the zinc displacement step (or zinc alloy displacement), the aluminum base material is exposed to a zinc displacement solution, which has basic ingredients of sodium hydride and zinc oxide. Consequently, a thin oxide film on the aluminum is removed, and zinc is separated and displaced on the newly exposed labile surface of the aluminum base material. As a result, zinc film covers the surface of the aluminum base material. If the zinc displacement process is repeated after the zinc film has been removed, the surface of the base material is made much more even.
After the complex pretreating stage, the base material is electroplated, which is commonly known. At the plating stage, the base material is immersed in a predetermined plating solution and a voltage is applied between electrodes. This forms an electroplated layer on the surface of the base material.
However, in the above plating method, the pretreating stage increases costs. It is also difficult to form a plated layer specifically on a limited surface area of the base material by the above method. When only the limited surface is to be plated, the rest of the surface is masked with insulating tape or other coating to expose only the limited area. The masking process further reduces efficiency.
Japanese Unexamined Patent Publication No. 8-104997 proposes a solution to this problem. According to the publication, plating fluid is squirted through a nozzle opening on the surface of the base material. Simultaneously, a plated layer is formed on any specified surface area of the base material by applying a voltage between the nozzle and the base material, which are electrically connected to each other by the plating fluid.
However, using this prior art method, it is difficult to form a plated layer with an even surface. Since the distal end of the nozzle has a cylindrical shape, the plating fluid is squirted through a circular opening. The flow velocity of plating fluid squirted from the nozzle is higher at the central region and lower in the region closer to the periphery. The resulting plated layer is formed with a thickness in accordance with the flow velocity of the plating fluid colliding against the surface of a base material. Therefore, the thickness of the plating layer is thicker at the central region and thinner in the region near the periphery.
Three-dimensional objects such as molds and dies are formed into a desirable shape by cutting or electron discharge. At the final stage, the formed products are ground by human hands. Ornaments such as bronze statues are formed by die casting, and the formed products are also ground.
However, these manufacturing methods are complicated and inefficient. The electron discharge method requires a large, expensive equipment. In these methods, final products are obtained by cutting off the base material, and the cut off base material is wasted.
When molds have relief areas, or the formed products have hollow shapes, die casting becomes difficult. In this case, many partial molds and slide cores are necessary. This complicates the molds and increases the manufacturing costs.